Meat tenderness is an important issue for consumers, and one which can influence demand sufficiently for an especially tender meat to command a premium price in the marketplace. The physiological change in muscle structure during the postmortem period is complex but clearly seems to be at least one factor in meat tenderness. The calpain/calpastatin system is an endogenous, calcium-dependent proteinase system, theorised to initiate in vivo muscle protein degradation. Calpastatin appears to inhibit calpain activity and therefore may be assumed to have a role in meat tenderness through the regulation of postmortem proteolysis. In particular, calpain is response for the breakdown of myofibril protein, which is closely related to meat tenderness.
It might therefore be suspected that calpastatin activity will be related to meat tenderness. Indeed, an increase in postmortem calpastatin activity has been correlated to reduced meat tenderness. Nevertheless, despite such observations, no clear link between the CAST gene, which encodes calpastatin, and meat tenderness has been established.
For example, Lonergan et al. (1995) undertook a restriction fragment length polymorphism analysis at CAST and failed to find an association with either calpastatin activity or tenderness in cross bred offspring of sires from eight breeds. Chung et al. (1999) measured calpastatin activity, Warner-Bratzler Shear Force and myofibril fragmentation index in forty-seven purebred Angus bulls. However, they concluded that “PCR single-strand conformation polymorphism analysis of the calpastatin gene was not useful for prediction of calpastatin activity, myofibril fragmentation index or meat tenderness”.
It is long known that one of the actions of lysyl oxidase (LOX) is to initiate crosslink formation at an early stage in collagen fibrillogenesis (e.g., Cronlund et al., 1985). The action of lysyl oxidase is intensively studied with hundreds of publications on a variety of aspects of its importance in cancer (Giampuzzi et al., 2001), the vasculature (Nellaiappan et al.) and other tissue and organ systems.
Variation at the gene itself has not been associated with differences in beef tenderness although LOX has always been seen as a strong candidate on biochemical grounds for a gene contributing to the collagen component of tenderness. Analysis of genetic linkage has implicated the genomic region that includes LOX in linkage analysis of family variation in adhesion and instron compression of the semitendinosis muscle (STADH and STIC; Drinkwater et al., 1999). However, LOX itself has not been associated with these measures of tenderness through the study of population associations.
Meat tenderness is a complicated trait because there are many sources of variation that affect postmortem meat tenderisation. Some of these are non-genetic effects such as the age of the beast, the nature of its feed, degree of stress prior to slaughter, carcass chilling, postmortem ageing time and cooking and testing methods. It has been suggested (e.g. Koohmaraie (1994)) that approximately 30% of the variation in tenderness in meat can be explained by additive gene effects within a single breed, and that approximately 70% of the variation is explained by environmental and non-additive gene effects. In the Lonergan study the cattle were slaughtered at just over 1 year of age (430 days), the sample contained only 83 animals of random peak-force values, and the sample consisted entirely of crosses between various taurine breeds. Likewise, in the Chung study purebred Angus bulls only 280 days of age were used. In addition, in neither study were the animales selected for extreme peak-force values, and it therefore seems that environmental and non-fixed genetic effects may have contributed to the failure to identify any genetic linkage between the CAST gene and meat tenderness.